Here is a zoomed out picture, still no name or mark. I can see the reason for scepticism but I do have a lot of British helmets already (only one other more conventional MKIII with the brown textured paint) and it was a 'In the hand purchase'. I would not of bothered you guys with it if I hadn't felt it was 'different'.

As the letters are vertical to the rim it will be a BMB made shell.
The standard paint finish for all military helmets in 41/42 was textured khaki green, I see no reason why they would not have used that same colour on the trial Mk3.
Early Mk3's 43/44 are textured khaki green.

Here's a turtle I picked up recently, can't find any date on the shell but the liner is marked BMB 111 1945 and has a lift the dot fastener, the inside of the helmet is painted smooth brown and the outside a textured olive green, the chin strap has what may be a service number on it. I take it this will be a late war or maybe post war mark IV ?.
cheers,
John.